This invention relates to the field of small particle size measurement and particle size distibution determinations in a stream of moving particles.
In numerous fields of current technical interest it is desirable to know the physical characteristics of small particles which are dispersed in a medium such as air or vacuum. Particle populations of this type may be of concern, for example, when liquids under pressure are ejected into the vacuum of outer space, or when air of imperfect purity is considered--especially at high velocity, as in a wind tunnel, or in the case of an aircraft moving through the atmosphere, or in the products of combustion of hydrocarbon fuels. Such particles can also be of concern in the gas and moisture exchange processes employed by living organic specimens, such as in human breathing, in the discharge of propelled projectile weapons, and in a diversity of other phenomena now under quantitative technical investigation.
Often the particles encountered in these applications have diameters of a few microns or a fraction of a micron and are therefore viewed or subjected to precise analysis only with carefully considered apparatus and techniques. The use of light scattered by small particles as a basis for observation and defining particle characteristics is well known in this art, and the mathematics relating to particle characterizations has been developed by workers such as Rayleigh, Lorenz, and Mie. The terms "Reyleigh scattering" and "Mie scattering" are commonly used in the diagnosis of particles residing in well-defined size and property classes. It is notable, however, that none of this prior particle diagnostic work has afforded the capability of analyzing particles existing in plural locations of a particle population through the use of coherent light excitation and polarized segregation of the particle scattered light into optical signal components.
The patent art, as is characterized by several patents in the particle diagnostic field, illustrates the absence of multiple point bipolar partial analysis capability. This patent art does include, however, several examples of particle measuring and size distribution apparatus. One example of such apparatus is found in the patent of Andreas H. Engel, U.S. Pat. No. 3,871,769, which discloses an apparatus for determining the diameters of small particles. The Engel apparatus is based on the use of coherent laser supplied light, a plurality of reflecting hologram filters which may employ the concept of Mie's scattering theory and a plurality of optical-to-electrical transducer detectors. In the Engel invention, laser light is focused at a plurality of physically separated focal points located along the axis of a Fresnel lens apparatus and particles of successively different size are detected at each of the successive focal points. In the Engel apparatus a good degree of agreement between the diameter of particles under consideration and the diameter contemplated in fabricating the respective light filter elements results in the generation of spherically shaped patterns which pass through a predetermined aperture with little loss; this situation is contrasted with a condition of disagreement between the filter and particle size wherein the light waves pass through the predetermined aperture with great loss of energy. The Engel invention is therefore based on the concept of identifying agreement between predetermined filter elements and the light waves resulting from a particle diffusion event. The Engel invention, further, must size a single particle at a time.
Another particle measurement apparatus, one intended for measuring particle size distributions, is shown in the patent of Hendricas G. Loos, U.S. Pat. No. 4,338,030, wherein the use of an array of different filter elements for again seeking a condition of match between filter characteristics and the particle size related diffusion of incident light energy is accomplished. The Loos apparatus employs a pulsed light source of preferably collimated white polarized light, uses a polarizer, and is arranged to seek a maximum light transfer through a filter which has a transmittance pattern precisely matching the Mie pattern of the incoming light. From the match between the Mie pattern and the filter characteristics, particle or drop size can be inferred. The Loos apparatus also contemplates the unraveling or separation of contributions by the Mie patterns belonging to different drop size classes.
Another particle measuring apparatus is shown in the patent of W.B. Underwood, U.S. Pat. No. 4,118,625, which concerns a nephelometer having a pulsed source of energy, preferably a pulse driven solid state laser device. The Underwood patent indicates a nephelometer to be a device wherein a beam of light traverses a liquid or other fluid for the purpose of detecting and measuring the scattering function of particles suspended in the fluid. The Underwood particle apparatus contemplates the use of infrared frequency energy and the use of a detector arrangement capable of ignoring ambient light conditions by way of pulsing the infrared energy of interest in performing the particle measurements.
A photometer for measuring the light scattered by particles in a hydrosol is disclosed in the patent of William R. McCluney, U.S. Pat. No. 4,053,229, which is entitled "2.degree./90.degree. Laboratory Scattering Photometer". The McCluney apparatus is intended for use in distinguishing between particles of high index of refraction with respect to water such as silica or calcium carbonate or other inorganic materials (and including organic skeletal materials, such as bone), and particles having a low index of refraction relative to water, such as organic tissue material--material wherein the light scattering ability tends to be at angles greater than 80.degree. on the one hand, and at angles of 1.degree. to 10.degree. on the other hand. The McCluney apparatus is further arranged to be convenient for use on vessels traveling in natural waters and industrial streams for performing a continuous particle analysis while the vessel is in motion. The McCluney apparatus also includes a beam splitting device which is used for monitoring the performance of the laser light source and employs photoelectric transducers that are preferably of the silicon diode rather than scanning or imaging type.